A tldr of my comment from the chapter’s comment section: I think C-Kira killed themselves because of one of these reasons
A) Their ‘compassion’ backfired on them, finally seeing that their ‘mercy kills’ were just murders after being called out,
B) L (Near) does not consider C-Kira as a worthy adversary, damaging their ego,
C) C-Kira could not accept that they weren’t being seen as Kira.
I think it’s a combination of all these, as the narrative of the story is about Near becoming his own version of L.
C-Kira could have been crushed to learn that they were nothing like Kira at all.